Exposure assessment, an essential component of environmental risk analysis, requires quantitative approaches for estimating the distribution and persistence of pollutants at Superfund sites. The most common pollutants are volatile organic compounds (VOCs). In addition to VOCs, perchlorate has recently been discovered as an important groundwater contaminant prevalent at several Superfund sites. This project provides an evaluation of human exposure through the development of innovative stimulation module that considers the geological heterogeneity of field sites. Improving upon existing predictive models requires improved understanding of how physical and chemical heterogeneity impact transport, how mineral and organic matter composition affect reactivity, how variations in oxidation status control both abiotic and biological transformations are coupled, and how each of these changes when chemical mixtures are present. The first goal of this project is to elucidate the physical, chemical, and biological processes that determine the transport, transformation and remediation of perchlorate and VOCs in soil and geologic sediments. The second goal is to integrate this process-level knowledge to predict contaminant movement and potential human exposure under complex field setting such as the Aeroject NPL site in CA. A third goal of the project is to explore techniques for remediation of hazardous waste. We will work on VOCs, perchlorate, and chemical mixtures. A particular focus will be on developing and applying new risk assessment procedures that include state-of-the-art transport and fate modeling approaches, thereby determining probabilistically the extent and duration of past human exposure to perchlorate near Aeroject. This project will investigate the basic physical, chemical, and biological processes affect the fate and transport of perchlorate, VOCs, and mixtures of these chemicals in the vadose zone and groundwater using laboratory batch and column experiments. It will also determine the effect of coupled processes on transport and fate and will investigate the effects of spatially varying processes and properties on field-scale behavior. This understanding of the fundamental processes will be used to explore the potential of in situ subsurface techniques as well as pump-and-treat for remediating perchlorate and TCE contamination at the Aeroject site. Using stochastic transport and fate stimulation models, the effects of multiple transport processes on potential human exposure levels at the field scale as well as human health effects will also be investigated with conjunction with other projects.